Winding of conductive tracks tightly around a carrier is often used in medical devices that can be used for stimulation and/or detection of neuron signals in the body (human and/or animal) by inserting them temporarily or permanently (implantable) in the body. The conductive tracks are for example required to relay signals between a sensor and/or stimulation device within the body and processing units and circuitry outside the body. The insertable/implantable part of the device needs to have certain dimensions that are usually rather small. The dimensions depend on the particular implementation. Specific known examples of such implementations are cochlea implants, but others such as (deep) brain stimulation devices, or muscle stimulation devices can be conceived. The current invention can be used for all such devices.
For example, the cochlea of the human ear contains hair cells that are essential to the perception of sound. Sound vibrations distort certain structures of the cochlea which in turn distort the hair cells. This initiates electrical impulses in the hair cells which are conveyed to the fibers of the auditory nerve and ultimately to the brain.
Some instances of human hearing loss are attributed to extensive destruction of the hair cells. When this occurs, though the structures of the cochlea may otherwise be substantially intact, and the auditory nerve may be partially or completely intact, the auditory response is significantly impaired or non-existent.
Cochlea implants directly stimulate the auditory nerves inside the inner ear. In a traditional cochlear implant system, a microphone acquires sound from the environment. The sound is then selectively filtered by a speech processor, using various filter bank strategies such as Fast Fourier Transforms, to divide the signal into different frequency bands. Once processed, the signal is then sent to a transmitter, a coil held in position by a magnet placed behind the external ear. This transmitter sends the processed signal to the internal device by electromagnetic induction.
Embedded in the skull, behind the ear is a receiver which converts the signal into electric impulses and sends them through an internal cable to electrodes. Conventional cochlear implants are made of multiple platinum electrodes or similar conductive material, connected to platinum wire and embedded in a silicone body. These electrodes then act to stimulate the auditory nerve fibers by generating an electric field when the electrical current is routed to them.
It is known that the cochlea implant should have a small insertion area so that the installation of the cochlear implant does not damage cochlear structures. Although perhaps not always necessary, implants for (deep) brain stimulation or other purposes are also likely to benefit from the smaller insertion area. This puts constraints on the cable dimensions of an insertable device and especially for the cochlea device.
One known design is based on a long strip of electrodes, which are then wound around a carrier to form a spiral strip cochlea implant. This provides the desired tubular shape for insertion into the cochlea. An example of this type of arrangement is for example disclosed in US 2012/0310258. The electrode design includes conductive and dielectric layers, to provide isolation of different electrode lines. The electrode design provides a limit to how tightly the strip can be wound, and this in turn provides a limit to how small the tube can be made and thus to the insertion area. In particular, bending the strip with a bend radius which is too small can result in damage of one or more of the layers forming the structure. Hence there is a need for an improved insertable/implantable device with a smaller insertion area